1. Field
This disclosure relates generally to electrical circuitry, and more specifically, to electrical circuitry for data conversion.
2. Related Art
Data converters are very useful for converting analog signals to digital signals, and for converting digital signals to analog signals. Many applications require data converters that have a high resolution, fast conversion time, allow a broad range of inputs, and yet are cost effective. Other data conversion features may also be important for various applications. It is thus important to be able to provide data converters that meet a wide variety of potentially conflicting criteria, while at the same time remain cost effective.
Analog MOS circuits such as switched-capacitor circuits often employed in analog to digital converters use charge to represent analog data. In such circuits, analog signals are converted from the voltage domain into the charge domain by applying a voltage to a capacitor through an MOS switch such as a field effect transistor. With the switch closed, an input voltage produces a charge on the top plate of the capacitor. If the switch is subsequently opened (by dropping the gate voltage below threshold), this charge will ideally remain on the capacitor. The principal limitations to the accuracy of this scheme come from the MOS switch. When the MOS switch is turned on, it generates thermal noise that causes random fluctuations in the device's drain current. The variations are continuously integrated by the capacitor. When the MOS switch is turned off, the integral of the noise current is “sampled” onto the capacitor. Thus an error component is added to the signal charge.
When the MOS switch turns off, another error source referred to as charge injection is caused by the mobile charge in the MOS switch's inversion layer, which is forced to leave the channel when the gate voltage changes. Any inversion charge that escapes to the data node can cause an additional error in the stored charge.